The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Internal combustion engines combust a mixture of air and fuel in cylinders and thereby produce drive torque. Combustion occurs within combustion chambers defined by the cylinders. Combustion may be initiated by a spark that supplies energy to the air-fuel mixture and thereby initiates combustion. Once initiated, combustion in the combustion chamber continues along a flame front for a period. Timing of the combustion may be generally controlled by controlling the timing of the spark. The timing of the spark may be controlled relative to a position of pistons that reciprocate within the cylinders and/or a rotational position of a crankshaft coupled to the pistons. For example, the timing of the spark may be controlled relative to a top-dead-center (TDC) position of the pistons. At TDC, the volume of the combustion chamber is at its smallest volume.
Spontaneous combustion of a portion of the air-fuel mixture within the combustion chamber may occur when a pressure wave created by the spark-initiated combustion travels faster than the flame front of the spark-initiated combustion. The pressure wave may result in a rapid pressure rise in end gases within the cylinders that causes the end gases to self-ignite (i.e., auto-ignite). Auto-ignition of the end gases may result in rapid combustion or detonation of the entire volume of end gases. Auto-ignition of the end gases results in a rapid release of heat that causes a rapid rise in cylinder pressure that may cause the cylinder pressure to resonate at natural acoustic frequencies of the combustion chamber. Sustained oscillations of the pressure waves may cause metal surfaces of the combustion chamber to vibrate and produce an audible sound referred to as engine knock. Thus, engine knock may occur as an impulse response of the combustion chamber in response to the rapid pressure rise caused by auto-ignition of the end gases and the resulting heat release.
Engines may be provided with a knock control system that detects the presence and intensity of engine knock. Several approaches have been developed to detect the presence of engine knock. In one approach, an accelerometer senses the mechanical vibration induced in the engine block structure as a result of the oscillating pressure wave in the combustion chamber. The energy of the mechanical vibration is used as an index of the intensity of the engine knock. The knock intensity may be determined by one of several methods, such as the integral of the square of the oscillation waveform or the maximum peak-to-peak value of the oscillations. In another approach, a pressure sensor senses cylinder pressure and thereby detects the oscillations in the cylinder pressure. Similar to the block structure vibration method, the energy of the pressure oscillations is used as an index of the knock intensity.
Based on the knock intensity, corrective action may be taken to inhibit engine knock. For example, engine spark timing may be retarded to slow down the rate of combustion to a rate that prevents the occurrence of engine knock. As such, knock control systems may be provided during engine development to assist in developing engine spark calibrations that reduce the occurrence of engine knock. In production engines, knock control systems may be provided to adjust spark timing in real time to a point where the engine knock disappears.